For the very first time, two spacecraft will fly in formation with millimeter precision

Apr 16, 2013

For the very first time, two spacecraft will fly in formation with sub-millimetre precision. Credit: ESA / P. Carril

Spanish industry is leading the Proba-3 mission, a world first in precise formation flying. This European Space Agency (ESA) project aims to demonstrate that two satellites can move as one single object with sub-millimetre precision. This configuration will enable the creation of enormous space telescopes with the lens and detector hundreds of metres apart.

"Proba-3 will be the first mission in which two spacecraft will fly through space as a single unit, pointing at selectable directions, and with sub-millimetre precision, in other words, relative position accuracy to within less than one millimetre," Salvador Llorente, director of this project in SENER, the first Spanish company to lead an ESA mission, explained to SINC.

There have been very few formation satellite missions up to now, such as the Swedish Prisma project, and only in the near Earth environment and with a level of precision of tens of centimetres.

The new mission includes two satellites weighing approximately 340 kg and 200 kg. They will be launched in 2017 –several launchers are being evaluated, including one from India and another from the US– and they will travel jointly attached together until they separate in a highly-eccentric orbit. Their nearest point, the perigee, will only be 600 km from the Earth. Every time they pass through this zone they will be in free flight, but under well controlled trajectories.

The operations associated with precise formation flight will take place on the most distant section of the orbit, the apogee, over 60,000 km away, as here the gravitational disturbances are minimised and do not complicate or make the manoeuvres too costly. The formation technology will be tested and the planned tests will be conducted in this region of the orbit.

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For the very first time, two spacecraft will fly in formation with millimetre precision. Credit: SENER

One of the relevant experiments including scientific application of Proba-3 will be blocking out the Sun with one of the craft in such a way that the other, 150 m away, can examine the Sun's corona in unprecedented detail.

The first satellite, the blocker, will create an artificial solar eclipse in order to facilitate its companion satellite, the coronograh, in gathering the data. A similar technique was already tried in 1975 on the Apollo-Soyuz mission.

"In any case, the primary objective of this mission is to validate the precision formation flight technology, and to be able to position both craft between 20 and 250 metres apart, yet always working together as if they formed a rigid structure," Llorente emphasised.

The researcher highlighted one possible application of this configuration: "If you wanted to build telescopes with long focal length, you could mount the lens on one of the satellites and the detector on the other, which was already proposed –for instance– in the case of the Xeus x-ray telescope." In this way you can avoid the need for large deployable structures and reduce the mass of the launch, apart from improving the position stability when compared with thermo-elastic distortion with such a large structure.

Proba-3 will also serve to validate several optical and laser sensors in addition to the algorithms required for future formation flight missions. Different experiments will be used to confirm that this system works properly, varying the distances between the satellites and their pointing direction.

Rendezvous tests, orbital approach manoeuvres between spacecraft in highly elliptical orbit, will also be performed and could be applied to missions to Mars. Specifically in the mission known as 'Mars Sample Return', which plans to pass a Martian rock from one craft to another.

In addition, researchers will test prevention and emergency systems with the activation of engines and other devices to avoid the risk of the satellites colliding, "a situation that entails the premature end to any mission," Llorente warned.

Although the ESA has entrusted SENER with the task of leading the project, other major partners taking part are Astrium CASA Espacio and GMV from Spain, as well as QinetiQ Space and Spacebel from Belgium. The mission's tracking station will be located in the Redu locality (Belgium), although adjustments between the two spacecraft can be programmed automatically.

The details of Proba-3 have been presented in journals such as 'Acta Astronautica' and at conferences such as the International Workshop on Satellite Constellation and Formation Flying (IWSCFF), held recently in Lisbon (Portugal). Researchers and companies from over a dozen countries are taking part in this third PRoject for On-Board Autonomy (PROBA), the ESA's advanced series of technology demonstration mini-satellites.

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User comments : 8

"If you wanted to build telescopes with long focal length, you could mount the lens on one of the satellites and the detector on the other, which was already proposed –for instance– in the case of the Xeus x-ray telescope."

I'm assuming they're going to use LIDAR or reference lasers between the two satellite in order for automated guidance to keep them oriented properly? Communications between Earth and the Satellite and back would have too big of a delay.

I'm not sure a telescope of that kind would be particularly useful, as you'd only be able to look directly at things which are exactly co-planar with the orbit*, and you'd only get a few seconds worth of exposure time per target object per orbit.

* Okay, a polar orbit of the Earth would allow a 3-d viewing area because you have 2-d from Earth's orbit, and a different plane from the polar orbit, would give 3-D, but polar orbits are also probably trickier for ground control. Additionally exposure time would be even less...

Telescopes like Hubble don't have that sort of alignment requirements, so it can stay focused on a single target object longer. Additionally, arrays of smaller telescopes can stay focused longer.

If you're going to use a system where one satellite is the lens and the other is a detector, you're talking about zero orientability, which means you can only look at whatever happens to be exactly in alignment of the two satellites, regardless of distance or importance. By the time you also figure our own planets, sun, and moon getting in the way of otherwise "lined up" shots, you may only get one opportunity every several years for looking at something you actually want to study.

The example of the Xeus telescope given in this article is not practical compared to an array telescope. Maybe something like that would be useful later, after we have a baseline of several space-based arrays or something, but for not it would seem to be pointless because it would be too specialized, and requires too much "coincidence" to be useful.

The example of the Xeus telescope given in this article is not practical compared to an array telescope. Maybe something like that would be useful later, after we have a baseline of several space-based arrays or something, but for not it would seem to be pointless because it would be too specialized, and requires too much "coincidence" to be useful.

We already have the technology for placing "array" telescopes into space. The purpose of this experiment to test the technology of placing a system with a long focal point into space by using separate satellites as the objective lens and the detector. Increasing the magnification with focal length rather than collector diameter.

As to being practical at only a few directions, that is not so. The two satellites are only meters (20m to 250m) apart, easy enough to reorient in almost any direction with current technology.

It would be similar to tasking of survalance satellites, only doing it in tandem.

"they will perform high precision range-rate measurements to precisely measure the changing distance between each other to within 1 micron, the width of a red blood cell, using a Ka-band instrument."http://www.univer...-orbits/

The difference between measuring changing distance and maintaining a fixed distance in space.

Interesting to know is if there is a fundamental limit to maintain a fixed distance between objects in motion positioned in space.

Yes, this is a very interesting topic of research. We will eventually need to do this to advance telescope technology.

Interesting to know is if there is a fundamental limit to maintain a fixed distance between objects in motion positioned in space

If there's a practical or fundamental limit, it's still possible to make up the difference with mechanical movement on either of the spacecraft, like moving the lense forward/backward to focus just like you do in a camera.

this is not about a general space based telescope like hubble or webb

This is primarilly a test mission, to see how well we can make it work. If it seems reasonable that we can make this work, or rather, 'when' it seems reasonable that we can make this work, someone will build an extremely long focal length telescope. We will eventually need to build one if we ever hope to see exoplanets clearly. With an adaptive lense, the potential is very cool.

We may have proscribed the fundamental limits ourselves. Math has instantaneous, continuous functions. With the physical we linearized this and say nature is discrete making the measure we have doable with the limits proscribed.Proponents of the abstract assert an impractical continuous space and a measure with arbitrary precision foreign to our reality.Thermal motion is a practical limit.

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